It is often desirable to correct tooth placement by employment of techniques facilitating space closure. Use of orthodontic appliances to control tooth movement during space closure is well known. Gaps may be reduced by reciprocal movement of teeth toward each other, or anterior retraction/posterior protraction of one or more teeth relative to one or more other teeth. Space closure is made difficult in that a simple force applied to a tooth may cause translation and/or rotation.
Under ideal conditions, a tooth can be translated through the alveolar crest, without unwanted rotation or tipping by applying a line of force at a tooth's center of resistance. Unfortunately, during space closure, forces are generally placed at a bracket which is typically positioned above the center of resistance. This generally produces a moment causing rotation of the tooth about a point somewhere between the root center of the tooth and its apex. It has been recognized that the movement resulting in tipping (M.sub.F) can be counteracted by applying a counter-moment (M.sub.C) More specifically, a couple is created which moves the "center of rotation" to infinity. Various "edgewise techniques," also known as "sliding mechanics," have been employed to provide for controlled tooth movements in which M.sub.F and M.sub.C are appropriately balanced. In one known technique, forces and moments are balanced by a centered "gable" placed in a main arch-wire, in combination with a force from an elastic or simple spring, to deliver a couple to overcome tipping during space closure. While a properly centered gable bend will produce equal and opposite moments of the roots, off-centered bends produce unequal moments which become further unbalanced as the gable bend is moved progressively away from the middle of the extraction site during space closure.
In another known technique, a squashed vertical loop with an edgewise wire, known as a "bull loop," is activated by opening the same the distance of a "thin dime". While bull loops can be effective in generating ideal "moment-to-force (M/F) ratios", they tend to be impractical since their range of activation is highly limited. Other edgewise techniques are discussed in Ricketts, R. Features of the Bioprooressive Therapy (No. 16) (1974) which is incorporated by reference.
"Segmented arch techniques," such as those promoted by J. Burstone, offer systems with controlled tooth movement and excellent M/F ratios, but contain numerous components which may be difficult to manipulate. Examples of orthodontic appliances of the segmented arch type may be found in Burstone U.S. Pat. No. 4,197,643. In using segmented arch techniques, no continous arch-wire is present between anterior and posterior segments of the teeth. Due to the elimination of the sliding resistance of the main arch, an overall reduction in force is realized. Without the presence of the arch-wire, however, nothing prevents anterior and posterior segments from leaving the occlusial plane due to errors in the M/F ratio between segments by unintentional or accidental means.
Other segmented arch techniques are discussed in the above-mentioned Ricketts reference. While the Ricketts cuspid retractors may be used with a utility arch, these cuspid retractors are believed not used with a main arch. Hence, the Ricketts system does not necessarily prevent unwanted rotation and particularly lacks the "fail-safe" approach of sliding mechanics. Moreover, the Ricketts appliance is gabled pursuant to insertion into brackets and, as mentioned above, the gable can become unbalanced as the gable bend is moved progressively away from the middle of the extraction site during space closure.
In view of the above-discussed prior art, there is a demand for a system that incorporates the theory of segmented arch techniques, which provide excellent M/F ratios, along with the fail-safe mechanism inherent to sliding mechanics. In view of the typical practitioner's reluctance to make drastic changes in orthodontic appliance systems, however, any proposed systems should be "user-friendly," i.e. adaptable for use with much of the armamentarium and appliances typically associated with either technique.